Common mode voltages, also known as ground loop voltages, occur when the ground potential at one point in a circuit is different than the ground potential at another point in the circuit. In addition, if there is a finite resistance between the two ground points then a common mode current, also known as a ground loop current, flows in the circuit. When common mode currents exist in a circuit, they are generally detrimental to system performance and can present a safety hazard to personnel.
Prior art sensing circuitry included a sensor for detecting the presence of some physical phenomena, such as temperature or pressure, and generating an electrical signal representative of the sensed phenomena. The signal representative of the sensed phenomena was then transmitted to a processing station, also a part of the sensing circuitry, for analysis. Typically, the sensor would be located in close proximity, for example, to a grounded piece of equipment or power line, which would induce a common mode voltage in the sensing circuitry. If there was a finite resistance between, for example, the equipment ground and the sensing circuit ground, a common mode current would flow into the sensing circuitry thereby distorting the signal representative of the sensed phenomena. One way in which the prior art sought to prevent common mode currents was by introducing isolation circuitry between the sensor and the processing station to increase the impedance through the sensing circuitry. However, complete isolation is not possible and there is always some common mode current flow into the sensing circuitry due to capacitive coupling across the isolation barrier. Consequently, even if the common mode current is reduced by the power and isolation circuitry to the point where it does not present a safety hazard, it may still be significant with respect to, and thereby distort, the signals produced by the measurement instruments.
Exemplary of prior art sensors is U.S. Pat. No. 4,118,663 to Barben for a Four Electrode Conductivity Sensor. The sensor includes a conductivity cell having a passageway and a first and second pair of electrodes. The passageway contains a sample of the fluid whose conductivity is to be determined. One electrode of each pair of electrodes provides a feedback signal to a drive circuit which maintains the other electrode of the pair at a defined potential. Consequently, there are first and second drive circuits. The first drive circuit maintains a potential of V, generally an alternating potential to prevent polarization of the electrodes, at the location of the first pair of electrodes. The second drive circuit, on the other hand, maintains a ground potential at the location of the second pair of electrodes. The current necessary to maintain the defined potential at the location of either electrode pair is indicative of the conductivity of the sample. Consequently, the current of either the first or second drive circuit is sampled to determine the conductivity of the fluid. Also included in the sensor are isolation capacitors which serve, if at all, to prevent direct common mode currents from distorting the conductivity measurement. However, these isolation capacitors do not prevent alternating common mode currents that are produced, for example, by power lines. Consequently, alternating common mode currents will still distort the conductivity measurements produced by this sensor.